Factor VIII is an important component of the intrinsic pathway of the blood coagulation cascade. In the circulation, factor VIII is mainly complexed to von Willebrand factor. Upon activation by thrombin, (Factor IIa), it dissociates from the complex to interact with factor IXa in the intrinsic coagulation cascade, which, in turn, activates factor X. Once removed from the von Willebrand factor complex, activated factor VIII is proteolytically inactivated by activated Protein C (APC), factor Xa, and factor IXa, and is quickly cleared from the blood stream. When complexed with normal von Willebrand factor protein, the half-life of factor VIII is approximately 12 hours, whereas in the absence of von Willebrand factor, the half-life of factor VIII is reduced to 2 hours (Tuddenham E G, et al., Br J Haematol. (1982) 52(2):259-267).
In hemophilia, the clotting of blood is disturbed by a lack of certain plasma blood clotting factors. Hemophilia A is a deficiency of factor VIII, and is a recessive sex-linked, X chromosome disorder that represents 80% of hemophilia cases. The standard of care for the management of hemophilia A is replacement therapy with recombinant factor VIII concentrates. Subjects with severe hemophilia A have circulating procoagulant factor VIII levels below 1-2% of normal, and are generally on prophylactic therapy with the aim of keeping factor VIII above 1% between doses, which can usually be achieved by giving factor VIII two to three times a week. Persons with moderately severe hemophilia (factor VIII levels of 2-5% of normal) constitute 25-30% hemophilia incidents and manifest bleeding after minor trauma. Persons with mild hemophilia A (factor VIII levels of 5-40% of normal) comprise 15-20% of all hemophilia incidents, and develop bleeding only after significant trauma or surgery.
The in vivo activity of exogenously supplied factor VIII is limited both by a short protein half-life and inhibitors that bind to the factor VIII and diminish or destroy hemostatic function.
Up to 30% of hemophilia A patients receiving exogenously-supplied factor VIII mount an IgG immune response towards factor VIII (Towfighi, F., et al. Comparative measurement of anti-factor VIII antibody by Bethesda assay and ELISA reveals restricted isotype profile and epitope specificity. Acta Haematol (2005) 114:84-90), which can result in the complete inhibition of its procoagulant activity and/or promote more rapid clearance of the factor VIII (Briet E et al. High titer inhibitors in severe haemophilia A. A meta-analysis based on eight long-term follow-up studies concerning inhibitors associated with crude or intermediate purity factor VIII products. Throm. Haemost. (1994) 72: 162-164). The IgG antibodies, called FVIII inhibitors, are primarily directed towards the A2, A3 and C2 domains (Scandella D et al. Localization of epitopes for human factor VIII inhibitor antibodies by immunoblotting and antibody neutralization. Blood (1989) 74:1618-1626), but can arise against the A1, B and C1 domains, as well. As such, treatment options for patients with FVIII inhibitors are limited.
Large proteins such as factor VIII are normally given intravenously so that the medicament is directly available in the blood stream. It has been previously demonstrated that an unmodified factor VIII injected intramuscularly yielded a maximum circulating level of only 1.4% of the normal plasma level (Pool et al, Ineffectiveness of Intramuscularly Injected Factor VIII Concentrate in Two Hemophilic Patients. New England J. Medicine (1966) 275(10):547-548). Formulations that could be administered other than by the intravenous route would greatly simplify their use, increase safety, and result in substantial cost savings.
Chemical modifications to a therapeutic protein can modify its in vivo clearance rate and subsequent serum half-life. One example of a common modification is the addition of a polyethylene glycol (PEG) moiety, typically coupled to the protein via an aldehyde or N-hydroxysuccinimide (NHS) group on the PEG reacting with an amine group (e.g. lysine side chain or the N-terminus). However, the conjugation step can result in the formation of heterogeneous product mixtures that require extraction, purification and/or other further processes, all of which inevitably affect product yield and quality control. Also, the pharmacologic function of coagulation factors may be hampered if amino acid side chains in the vicinity of its binding site become modified by the PEGylation process. Other approaches include the genetic fusion of an Fc domain to the therapeutic protein, which increases the size of the therapeutic protein, hence reducing the rate of clearance through the kidney. In some cases, the Fc domain confers the ability to bind to, and be recycled from lysosomes by the FcRn receptor, resulting in increased pharmacokinetic half-life. Unfortunately, the Fc domain does not fold efficiently during recombinant expression, and tends to form insoluble precipitates known as inclusion bodies. These inclusion bodies must be solubilized and functional protein must be renatured from the misfolded aggregate, which is a time-consuming, inefficient, and expensive process.